I have put together a guide which people may be interested in:
https://github.com/tomsmalley/custom-topre-guide
It's based on work I have done on my projects I've posted here. I hope to get more people hacking these together, and want to update that repo to become a more cohesive guide as time goes on. If you know how to design a circuit in KiCad and can code for your microcontroller of choice (you can even use a Teensy, I used that in initial development), you can make one! The methods outlined there seem to work well.